1 /*
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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23 */
24
25 #ifndef SHARE_OOPS_MARKWORD_HPP
26 #define SHARE_OOPS_MARKWORD_HPP
27
28 #include "metaprogramming/primitiveConversions.hpp"
29 #include "oops/compressedKlass.hpp"
30 #include "oops/oopsHierarchy.hpp"
31 #include "runtime/globals.hpp"
32
33 #include <type_traits>
34
35 // The markWord describes the header of an object.
36 //
37 // Bit-format of an object header (most significant first, big endian layout below):
38 //
39 // 32 bits:
40 // --------
41 // hash:25 ------------>| age:4 self-fwd:1 lock:2 (normal object)
42 //
43 // 64 bits:
44 // --------
45 // unused:22 hash:31 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
46 //
47 // 64 bits (with compact headers):
48 // -------------------------------
49 // unused:32 klass:19 hashctrl:2 -->| unused_gap:4 age:4 self-fwd:1 lock:2 (normal object)
50 //
51 // Note: klass occupies bits 13-31 (19 bits), hashctrl occupies bits 11-12 (2 bits)
52 //
53 // - hash contains the identity hash value: largest value is
54 // 31 bits, see os::random(). Also, 64-bit vm's require
55 // a hash value no bigger than 32 bits because they will not
56 // properly generate a mask larger than that: see library_call.cpp
57 //
58 // - With +UseCompactObjectHeaders:
59 // hashctrl bits indicate if object has been hashed:
60 // 00 - never hashed
61 // 01 - hashed, but not expanded by GC: will recompute hash
62 // 10 - not hashed, but expanded; special state used only by CDS to deal with scratch classes
63 // 11 - hashed and expanded by GC, and hashcode has been installed in hidden field
64 //
65 // When identityHashCode() is called, the transitions work as follows:
66 // 00 - set the hashctrl bits to 01, and compute the identity hash
67 // 01 - recompute idendity hash. When GC encounters 01 when moving an object, it will allocate an extra word, if
68 // necessary, for the object copy, and install 11.
69 // 11 - read hashcode from field
70 //
71 // - the two lock bits are used to describe three states: locked/unlocked and monitor.
72 //
73 // [ptr | 00] locked ptr points to real header on stack (stack-locking in use)
74 // [header | 00] locked locked regular object header (fast-locking in use)
75 // [header | 01] unlocked regular object header
76 // [ptr | 10] monitor inflated lock (header is swapped out, UseObjectMonitorTable == false)
77 // [header | 10] monitor inflated lock (UseObjectMonitorTable == true)
78 // [ptr | 11] marked used to mark an object
79 // [0 ............ 0| 00] inflating inflation in progress (stack-locking in use)
80 //
81 // We assume that stack/thread pointers have the lowest two bits cleared.
82 //
83 // - INFLATING() is a distinguished markword value of all zeros that is
84 // used when inflating an existing stack-lock into an ObjectMonitor.
85 // See below for is_being_inflated() and INFLATING().
86
87 class BasicLock;
88 class ObjectMonitor;
89 class JavaThread;
90 class outputStream;
91
92 class markWord {
93 private:
94 uintptr_t _value;
95
96 public:
97 explicit markWord(uintptr_t value) : _value(value) {}
98
99 markWord() = default; // Doesn't initialize _value.
100
101 // It is critical for performance that this class be trivially
102 // destructable, copyable, and assignable.
103 ~markWord() = default;
104 markWord(const markWord&) = default;
105 markWord& operator=(const markWord&) = default;
106
107 static markWord from_pointer(void* ptr) {
108 return markWord((uintptr_t)ptr);
109 }
110 void* to_pointer() const {
111 return (void*)_value;
112 }
113
114 bool operator==(const markWord& other) const {
115 return _value == other._value;
116 }
117 bool operator!=(const markWord& other) const {
118 return !operator==(other);
119 }
120
121 // Conversion
122 uintptr_t value() const { return _value; }
123 uint32_t value32() const { return (uint32_t)_value; }
124
125 // Constants
126 static const int age_bits = 4;
127 static const int lock_bits = 2;
128 static const int self_fwd_bits = 1;
129 static const int max_hash_bits = BitsPerWord - age_bits - lock_bits - self_fwd_bits;
130 static const int hash_bits = max_hash_bits > 31 ? 31 : max_hash_bits;
131 static const int unused_gap_bits = LP64_ONLY(4) NOT_LP64(0); // Reserved for Valhalla.
132 static const int hashctrl_bits = 2;
133
134 static const int lock_shift = 0;
135 static const int self_fwd_shift = lock_shift + lock_bits;
136 static const int age_shift = self_fwd_shift + self_fwd_bits;
137 static const int hash_shift = age_shift + age_bits + unused_gap_bits;
138 static const int hashctrl_shift = age_shift + age_bits + unused_gap_bits;
139
140 static const uintptr_t lock_mask = right_n_bits(lock_bits);
141 static const uintptr_t lock_mask_in_place = lock_mask << lock_shift;
142 static const uintptr_t self_fwd_mask = right_n_bits(self_fwd_bits);
143 static const uintptr_t self_fwd_mask_in_place = self_fwd_mask << self_fwd_shift;
144 static const uintptr_t age_mask = right_n_bits(age_bits);
145 static const uintptr_t age_mask_in_place = age_mask << age_shift;
146 static const uintptr_t hash_mask = right_n_bits(hash_bits);
147 static const uintptr_t hash_mask_in_place = hash_mask << hash_shift;
148 static const uintptr_t hashctrl_mask = right_n_bits(hashctrl_bits);
149 static const uintptr_t hashctrl_mask_in_place = hashctrl_mask << hashctrl_shift;
150 static const uintptr_t hashctrl_hashed_mask_in_place = ((uintptr_t)1) << hashctrl_shift;
151 static const uintptr_t hashctrl_expanded_mask_in_place = ((uintptr_t)2) << hashctrl_shift;
152
153 #ifdef _LP64
154 // Used only with compact headers:
155 // With UseCompactObjectHeaders: We store the (narrow) Klass* in bits 13-31 (19 bits total).
156 // Without UseCompactObjectHeaders: Klass* is stored separately in object header, not in markword.
157
158 // These are for bit-precise extraction of the narrow Klass* from the markword (UseCompactObjectHeaders only)
159 //
160 // Bit position summary for UseCompactObjectHeaders:
161 // Bits 0- 1: lock (2 bits)
162 // Bit 2 : self-fwd (1 bit)
163 // Bits 3- 6: age (4 bits)
164 // Bits 7-10: unused_gap (4 bits)
165 // Bits 11-12: hashctrl (2 bits) - hash control state
166 // Bits 13-31: klass (19 bits) - narrow klass pointer
167 // Bits 32-63: unused (32 bits)
168 //
169 // Without UseCompactObjectHeaders, klass is stored separately in object header
170 static constexpr int klass_shift = hashctrl_shift + hashctrl_bits;
171 static constexpr int klass_bits = 19;
172 static constexpr uintptr_t klass_mask = right_n_bits(klass_bits);
173 static constexpr uintptr_t klass_mask_in_place = klass_mask << klass_shift;
174 #endif
175
176
177 static const uintptr_t locked_value = 0;
178 static const uintptr_t unlocked_value = 1;
179 static const uintptr_t monitor_value = 2;
180 static const uintptr_t marked_value = 3;
181 static const uintptr_t forward_expanded_value = 0b111;
182
183 static const uintptr_t no_hash = 0 ; // no hash value assigned
184 static const uintptr_t no_hash_in_place = (uintptr_t)no_hash << hash_shift;
185 static const uintptr_t no_lock_in_place = unlocked_value;
186
187 static const uint max_age = age_mask;
188
189 // Creates a markWord with all bits set to zero.
190 static markWord zero() { return markWord(uintptr_t(0)); }
191
192 // lock accessors (note that these assume lock_shift == 0)
193 bool is_locked() const {
194 return (mask_bits(value(), lock_mask_in_place) != unlocked_value);
195 }
196 bool is_unlocked() const {
197 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
198 }
199 bool is_marked() const {
200 return (value() & (self_fwd_mask_in_place | lock_mask_in_place)) > monitor_value;
201 }
202 bool is_forwarded() const {
203 // Returns true for normal forwarded (0b011) and self-forwarded (0b1xx).
204 return mask_bits(value(), lock_mask_in_place | self_fwd_mask_in_place) >= static_cast<intptr_t>(marked_value);
205 }
206 bool is_neutral() const { // Not locked, or marked - a "clean" neutral state
207 return (mask_bits(value(), lock_mask_in_place) == unlocked_value);
208 }
209
210 markWord set_forward_expanded() {
211 assert((value() & (lock_mask_in_place | self_fwd_mask_in_place)) == marked_value, "must be normal-forwarded here");
212 return markWord(value() | forward_expanded_value);
213 }
214 bool is_forward_expanded() {
215 return (value() & (lock_mask_in_place | self_fwd_mask_in_place)) == forward_expanded_value;
216 }
217
218 // Special temporary state of the markWord while being inflated.
219 // Code that looks at mark outside a lock need to take this into account.
220 bool is_being_inflated() const { return (value() == 0); }
221
222 // Distinguished markword value - used when inflating over
223 // an existing stack-lock. 0 indicates the markword is "BUSY".
224 // Lockword mutators that use a LD...CAS idiom should always
225 // check for and avoid overwriting a 0 value installed by some
226 // other thread. (They should spin or block instead. The 0 value
227 // is transient and *should* be short-lived).
228 // Fast-locking does not use INFLATING.
229 static markWord INFLATING() { return zero(); } // inflate-in-progress
230
231 // Should this header be preserved during GC?
232 bool must_be_preserved() const {
233 return UseCompactObjectHeaders ? !is_unlocked() : (!is_unlocked() || !has_no_hash());
234 }
235
236 // WARNING: The following routines are used EXCLUSIVELY by
237 // synchronization functions. They are not really gc safe.
238 // They must get updated if markWord layout get changed.
239 markWord set_unlocked() const {
240 return markWord(value() | unlocked_value);
241 }
242 bool has_locker() const {
243 assert(LockingMode == LM_LEGACY, "should only be called with legacy stack locking");
244 return (value() & lock_mask_in_place) == locked_value;
245 }
246 BasicLock* locker() const {
247 assert(has_locker(), "check");
248 return (BasicLock*) value();
249 }
250
251 bool is_fast_locked() const {
252 assert(LockingMode == LM_LIGHTWEIGHT, "should only be called with new lightweight locking");
253 return (value() & lock_mask_in_place) == locked_value;
254 }
255 markWord set_fast_locked() const {
256 // Clear the lock_mask_in_place bits to set locked_value:
257 return markWord(value() & ~lock_mask_in_place);
258 }
259
260 bool has_monitor() const {
261 return ((value() & lock_mask_in_place) == monitor_value);
262 }
263 ObjectMonitor* monitor() const {
264 assert(has_monitor(), "check");
265 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
266 // Use xor instead of &~ to provide one extra tag-bit check.
267 return (ObjectMonitor*) (value() ^ monitor_value);
268 }
269 bool has_displaced_mark_helper() const {
270 intptr_t lockbits = value() & lock_mask_in_place;
271 if (LockingMode == LM_LIGHTWEIGHT) {
272 return !UseObjectMonitorTable && lockbits == monitor_value;
273 }
274 // monitor (0b10) | stack-locked (0b00)?
275 return (lockbits & unlocked_value) == 0;
276 }
277 markWord displaced_mark_helper() const;
278 void set_displaced_mark_helper(markWord m) const;
279 markWord copy_set_hash(intptr_t hash) const {
280 assert(!UseCompactObjectHeaders, "Do not use with compact i-hash");
281 uintptr_t tmp = value() & (~hash_mask_in_place);
282 tmp |= ((hash & hash_mask) << hash_shift);
283 return markWord(tmp);
284 }
285 // it is only used to be stored into BasicLock as the
286 // indicator that the lock is using heavyweight monitor
287 static markWord unused_mark() {
288 return markWord(marked_value);
289 }
290 // the following two functions create the markWord to be
291 // stored into object header, it encodes monitor info
292 static markWord encode(BasicLock* lock) {
293 return from_pointer(lock);
294 }
295 static markWord encode(ObjectMonitor* monitor) {
296 assert(!UseObjectMonitorTable, "Lightweight locking with OM table does not use markWord for monitors");
297 uintptr_t tmp = (uintptr_t) monitor;
298 return markWord(tmp | monitor_value);
299 }
300
301 markWord set_has_monitor() const {
302 return markWord((value() & ~lock_mask_in_place) | monitor_value);
303 }
304
305 // used to encode pointers during GC
306 markWord clear_lock_bits() const { return markWord(value() & ~(lock_mask_in_place | self_fwd_mask_in_place)); }
307
308 // age operations
309 markWord set_marked() { return markWord((value() & ~lock_mask_in_place) | marked_value); }
310 markWord set_unmarked() { return markWord((value() & ~lock_mask_in_place) | unlocked_value); }
311
312 uint age() const { return (uint) mask_bits(value() >> age_shift, age_mask); }
313 markWord set_age(uint v) const {
314 assert((v & ~age_mask) == 0, "shouldn't overflow age field");
315 return markWord((value() & ~age_mask_in_place) | ((v & age_mask) << age_shift));
316 }
317 markWord incr_age() const { return age() == max_age ? markWord(_value) : set_age(age() + 1); }
318
319 // hash operations
320 intptr_t hash() const {
321 assert(!UseCompactObjectHeaders, "only without compact i-hash");
322 return mask_bits(value() >> hash_shift, hash_mask);
323 }
324
325 bool has_no_hash() const {
326 if (UseCompactObjectHeaders) {
327 return !is_hashed();
328 } else {
329 return hash() == no_hash;
330 }
331 }
332
333 inline bool is_hashed_not_expanded() const {
334 assert(UseCompactObjectHeaders, "only with compact i-hash");
335 return (value() & hashctrl_mask_in_place) == hashctrl_hashed_mask_in_place;
336 }
337 inline markWord set_hashed_not_expanded() const {
338 assert(UseCompactObjectHeaders, "only with compact i-hash");
339 return markWord((value() & ~hashctrl_mask_in_place) | hashctrl_hashed_mask_in_place);
340 }
341
342 inline bool is_hashed_expanded() const {
343 assert(UseCompactObjectHeaders, "only with compact i-hash");
344 return (value() & hashctrl_mask_in_place) == (hashctrl_hashed_mask_in_place | hashctrl_expanded_mask_in_place);
345 }
346 inline markWord set_hashed_expanded() const {
347 assert(UseCompactObjectHeaders, "only with compact i-hash");
348 return markWord((value() & ~hashctrl_mask_in_place) | (hashctrl_hashed_mask_in_place | hashctrl_expanded_mask_in_place));
349 }
350
351 // This is a special hashctrl state (11) that is only used
352 // during CDS archive dumping. There we allocate 'scratch mirrors' for
353 // each real mirror klass. We allocate those scratch mirrors
354 // in a pre-extended form, but without being hashed. When the
355 // real mirror gets hashed, then we turn the scratch mirror into
356 // hashed_moved state, otherwise we leave it in that special state
357 // which indicates that the archived copy will be allocated in the
358 // unhashed form.
359 inline bool is_not_hashed_expanded() const {
360 assert(UseCompactObjectHeaders, "only with compact i-hash");
361 return (value() & hashctrl_mask_in_place) == hashctrl_expanded_mask_in_place;
362 }
363 inline markWord set_not_hashed_expanded() const {
364 assert(UseCompactObjectHeaders, "only with compact i-hash");
365 return markWord((value() & ~hashctrl_mask_in_place) | hashctrl_expanded_mask_in_place);
366 }
367 // Return true when object is either hashed_moved or not_hashed_moved.
368 inline bool is_expanded() const {
369 assert(UseCompactObjectHeaders, "only with compact i-hash");
370 return (value() & hashctrl_expanded_mask_in_place) != 0;
371 }
372 inline bool is_hashed() const {
373 assert(UseCompactObjectHeaders, "only with compact i-hash");
374 return (value() & hashctrl_hashed_mask_in_place) != 0;
375 }
376
377 inline markWord copy_hashctrl_from(markWord m) const {
378 if (UseCompactObjectHeaders) {
379 return markWord((value() & ~hashctrl_mask_in_place) | (m.value() & hashctrl_mask_in_place));
380 } else {
381 return markWord(value());
382 }
383 }
384
385 inline Klass* klass() const;
386 inline Klass* klass_or_null() const;
387 inline Klass* klass_without_asserts() const;
388 inline narrowKlass narrow_klass() const;
389 inline markWord set_narrow_klass(narrowKlass narrow_klass) const;
390
391 #ifdef _LP64
392 inline int array_length() { return checked_cast<int>(value() >> 32); }
393 #endif
394
395 // Prototype mark for initialization
396 static markWord prototype() {
397 if (UseCompactObjectHeaders) {
398 return markWord(no_lock_in_place);
399 } else {
400 return markWord(no_hash_in_place | no_lock_in_place);
401 }
402 }
403
404 // Debugging
405 void print_on(outputStream* st, bool print_monitor_info = true) const;
406
407 // Prepare address of oop for placement into mark
408 inline static markWord encode_pointer_as_mark(void* p) { return from_pointer(p).set_marked(); }
409
410 // Recover address of oop from encoded form used in mark
411 inline void* decode_pointer() const { return (void*)clear_lock_bits().value(); }
412
413 inline bool is_self_forwarded() const {
414 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
415 // Match 100, 101, 110 but not 111.
416 return mask_bits(value() + 1, (lock_mask_in_place | self_fwd_mask_in_place)) > 4;
417 }
418
419 inline markWord set_self_forwarded() const {
420 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
421 return markWord(value() | self_fwd_mask_in_place);
422 }
423
424 inline markWord unset_self_forwarded() const {
425 NOT_LP64(assert(LockingMode != LM_LEGACY, "incorrect with LM_LEGACY on 32 bit");)
426 return markWord(value() & ~self_fwd_mask_in_place);
427 }
428
429 inline oop forwardee() const {
430 return cast_to_oop(decode_pointer());
431 }
432 };
433
434 // Support atomic operations.
435 template<>
436 struct PrimitiveConversions::Translate<markWord> : public std::true_type {
437 typedef markWord Value;
438 typedef uintptr_t Decayed;
439
440 static Decayed decay(const Value& x) { return x.value(); }
441 static Value recover(Decayed x) { return Value(x); }
442 };
443
444 #endif // SHARE_OOPS_MARKWORD_HPP